1. Field of the Invention
The present invention relates to a lithium battery, and more particularly, to an organic electrolytic solution that suppresses a side reaction on the surface of an anode to thereby maintain reliability of charge/discharge reactions.
2. Description of the Related Art
Portable electronic devices, such as video cameras, cellular phones, notebook computers, etc., are becoming more lightweight and increasingly improve in performance. As a result, more research is being conducted into batteries for use as power supplies in such portable devices. In particular, chargeable lithium secondary batteries have 3 times as much energy density per unit weight as conventional lead storage batteries, nickel-cadmium batteries, nickel-hydrogen batteries, nickel-zinc batteries, etc., and can be rapidly charged. Thus these chargeable lithium secondary batteries are being actively researched.
In a lithium ion battery, transition metal compounds such as LiNiO2, LiCoO2, LiMn2O4, LiFePO4, LiNixCo1-xO2 (where x=1), Ni1-x-yCoxMnyO2 (where 0≦x≦0.5, 0≦y≦0.5) or oxides thereof containing lithium can be used as cathode active materials and lithium metals, lithium alloys, carbonaceous materials, graphites, etc. can be used as anode active materials.
Electrolytes can be classified as liquid electrolytes and solid electrolytes. When a liquid electrolyte is used, many safety problems arise, such as the risk of fire due to leakage of the electrolytic solution and the breakage of the battery due to vaporization of the electrolytic solution. To address these problems, a solid electrolyte has been proposed for use in place of the liquid electrolyte. Solid electrolytes do not leak and can be easily processed. A lot of research has been conducted into solid electrolytes, such as solid polymer electrolytes. Known solid polymer electrolytes can be classified as complete solid electrolytes containing no organic electrolytic solution and gel-type electrolytes containing an organic electrolytic solution.
Since lithium batteries generally operate at high operating voltages, conventional aqueous electrolytic solutions cannot be used. This is because lithium contained in the anode reacts vigorously with the aqueous solution. Thus, organic electrolytic solutions in which lithium salts are dissolved in organic solvents are used in lithium batteries. To that end, organic solvents having high ionic conductivity and dielectric constants and low viscosity may be used. Since it is difficult to obtain a single organic solvent satisfying these requirements, a mixed solvent is used, which includes an organic solvent having a high dielectric constant and an organic solvent having low viscosity, etc.
In lithium secondary batteries, a passivation layer, such as a solid electrolyte interface (“SEI”) film, forms on the negative electrode surface upon initial charging through a reaction of carbon in the anode with the electrolytic solution. The SEI film enables the battery to be stably charged and discharged without further decomposition of the electrolytic solution. Also, the SEI film acts as an ion tunnel through which only lithium ions pass and prevents co-intercalation of organic solvent, which solvates lithium ions and moves with the lithium ions into the carbon anode, thereby preventing a breakdown of the anode structure.
However, upon initial charging, gas is generated inside the battery due to decomposition of the carbonate-based organic solvent during formation of the SEI film. This gas generation results in swelling of the battery. When the lithium battery is stored at high temperature after charging, the passivation layer gradually breaks down due to increases over time in electrochemical energy and thermal energy. This break down of the passivation layer exposes the anode surface and results in increases in the amount of gas generated. Due to the generation of gas, the internal pressure of the battery increases, causing a deformation in the center of a side of the battery. Such deformation may be swelling of a rectangular lithium polymer battery in a certain direction. The increase in internal battery pressure results in a local difference in adherence between electrode plates, thereby reducing the performance and safety of the battery and increasing the difficulties in mounting a set of lithium secondary-batteries.
To address the above problems, a surfactant has been added to the electrolytic solution and adsorbed to the anode to prevent the electrolytic solution from directly contacting the negative electrode, thereby suppressing a side reaction. General cationic, anionic, and nonionic surfactants have been used.
Nonionic surfactants have been proposed for use in the anode to help the re-impregnation of the electrolyte into the anode. During charging and discharging, the electrolyte is squeezed out of the anode due to variations in the volume of the anode. By aiding the re-impregnation of the electrolyte into the anode, the nonionic surfactant prevents depletion of the electrolyte at the anode.
However, most conventional surfactants have structures including a single hydrophobic segment connected to a single hydrophilic segment. When these surfactants are used in the interface between the anode (which mainly includes carbon-based materials having repeating aromatic group structures) and the electrolyte (which mainly includes a carbonate-based solvent), the surfactants do not display good surface active properties due to the differences between the structure of the surfactants and the structures of the anode and electrolyte.
Thus, conventional surfactants have limited use both as barriers between the negative electrode and the electrolyte and as assistants for re-impregnation of the electrolyte into the anode. Therefore, a need exists for an electrolyte including a new surfactant having improved surface active properties in nonaqueous environments.